Your memory about the class mechanics is completely wrong. Tanks did more threat and had way better defensives than in WotLK, and EVERY Dps spec worked a lot better mechanically than its counterpart did in WotLK. Dots scaling with Haste was possibly the biggest mechanical improvement classwise in the entire history of the game. Cata was the first expansion with anything close to proper classbalance (outputwise). Casters most certainly did not go oom unless you forgot to use your manaregen spells (Shadowfiend, Evocation, Life Tap etc,etc).
The only roles that had a harder time than in WotLK, were Healers, and that was only because of mana issues the first few months. Their actual toolkit, spells and cooldowns were a lot stronger than in WotLK.
The first half of Cata was the perfect expansion imo. Out of everything they did since then, the only thing I would have added was M+ and forced MS loot prio (I believe everybody could still roll for everything that dropped at that point, unless that was WotLK, I know MS prio was there in MoP atleast). Yes, MoP style grploot is vastly superior to PL.
The latter half of Cata, aka 4.3, is the reason why the expansion is remembered as one of the worst today. 4.3 was probably the single laziest Major patch Blizz has ever done, possibly only beaten by the Selfie patch in WoD. Dragon Soul was a boring raid with tons of reused models turned slightly bigger as bosses, and the 3 new dungeons were complete dogshit compared to the both the original Cata 5mans and the ZA/ZG remakes, both of which were irrelevant at this point.